Towel Rail - Electric Element

[mike2016]

I've two Towel Rails to commission, Electric only. I'd purchased some Screwfix Flomasta 600w basic heating elements for them but then bought two more intelligent ones with built in controls (no wifi or bluetooth). The only issue is I would really like to put Shelly relays against these as the intelligent one only offers a 2 hour shortest period and I don't need to heat the room, only the towels. So I'd like to have control over shorter periods using a Home Assistant Schedule ideally. 

I'm back looking into this and I think I might use the dumb Flomasta ones (620TF), I just wanted to check they are "safe" to use and won't boil the water in the towel rad?! It's meant to heat up from 40-70oC and is "self limiting" and I don't want it relying on me to turn it off if I get my automation wrong. Would I be better looking for thermostatically controlled ones? Thanks!